The interaction of estrogen receptor (ER) complexes with nuclei and chromatin, and the effect of this interaction on gene expression will be investigated in vitro. Attention will be directed to defining specific differences in the interaction when estrogen concentration is raised from physiological to pharmacological levels or when the anti-estrogen tamoxifen is present. The purpose of this investigation will be to elucidate some of the basic steps by which ER can induce changes in gene expression, and then to explain why high estrogen levels or use of tamoxifen results in a blockage of the normal response of cells to estrogen. Specifically, conditions will be established which maximize the interaction of calf ER with nuclear material from calf hormone target tissues, compared to material from non-target tissues. It will be determined whether there are more than one class of binding affinities of ER to nuclear material, under these conditions, and dissociation constants will be measured. Breakdown or turnover of the ER in nuclei will be measured. In order than changes induced in gene expression can be measured in a laboratory setting, a different target tissue must be chosen. The R3230AC rat mammary tumor is a homogeneous, estrogen responsive tissue. Ovariectomized rats bearing this tumor will be exposed to physiological and high levels of estrogen, or tamoxifen, and the tumors will be removed at short times later. Effects of the hormone on chromatin structure will be determined by circular dichroism and RNA synthesis assay in vitro. Overall, results will reveal the effects of hormone type and level of interaction of ER with chromatin, as well as those hormone-induced structural changes that occur in the chromatin.